Primer for powder coating comprising (a) diene polymer (b) MgO and (c) cobalt or manganese ion

ABSTRACT

A primer composition for use in the fluidized-bed dip-coating of thermoplastic resin powder, comprising a polymeric compound formed by polymerization of a diene compound, admixed with either magnesium oxide alone or magnesium oxide together with cobalt ion and/or manganese ion.

This is a division of application Ser. No. 971,242, filed Dec. 20, 1978,now U.S. Pat. No. 4,268,579, which in turn is a continuation ofapplication Ser. No. 665,374, filed Mar. 10, 1976 now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a primer for use in the fluidized-beddip-coating of thermoplastic resin powder.

2. Description of Prior Art

Powder coating methods are considered excellent from the viewpoints ofresource savings, prevention of environmental pollution and excellentcoating properties.

These powder coating methods are divided into two types according to theapplication field, namely, decorative coating and functional coating forimparting to the coated object such properties as corrosion resistance,electric insulation, abrasion resistance and weatherability. Forfunctional coating, the process of fluidized-bed dip-coating ofthermoplastic resin powder is widely employed. In this process, thecoating is applied in a thickness of several hundred microns and theusing conditions employed are generally very severe. Accordingly, inaddition to degradation of the coating, the frequent occurrence ofpeeling is a great problem in the art of fluidized-bed dip-coating ofthermoplastic resin powder.

This invention relates to a primer for preventing peeling of athermoplastic resin coating, formed by the fluidized-bed dip-coating ofthermoplastic resin powder, from a metal substrate. More particularly,this invention relates to a primer having a good resistance to hot waterwhich otherwise promotes extremely the peeling of such a coating from ametal substrate.

A compound formed by polymerizing a diene compound has already been usedas a primer for a thermoplastic resin powder, especially nylon powder.However, as is seen from the results of Comparative Example 1 given inTable 1 hereinafter, the boiling water resistance of a polymeric primerformed by polymerizing a diene compound is about 20 to about 30 hours,and this value cannot be said to be a sufficient value.

We performed research with a view to improving the boiling waterresistance of polymeric primers formed by polymerizing diene compounds,but all attempts based on conventional chemical knowledge resulted infailure. For example, substances considered to have some influence onthe chemical reactions of polymeric compounds formed by polymerizingdiene compounds, such as radical initiators, oxidation promotors,antioxidants and peroxide decomposing agents, were found to have noeffect of improving the boiling water resistance. Further, substanceshaving an influence on the interfacial phenomenon, such as silicone oilsand silane coupling agents, were found to have no effect.

Still further, fillers generally incorporated in primers or coatings,such as aluminum powder, titanium oxide, talc, hollow glass beads, zincpowder, graphite powder, aluminum oxide powder, antimony oxide anddiatomaceous earth, were found to have no effect at all.

DETAILED DESCRIPTION OF INVENTION

We discovered, contrary to what might have been expected by conventionalchemical knowledge, that the addition of magnesium oxide, which isreadily converted to magnesium hydroxide by contact with boiling water,is very effective for improving the boiling water resistance ofpolymeric primers formed by polymerizing diene compounds. As is seenfrom the results of Example 4 given in Table 1 hereinafter, by theaddition of magnesium oxide, the boiling water resistance of the primeris improved by at least 10 times. Use of the crystalline, very bulkyform of magnesium oxide commonly referred to as "light" magnesium oxideis especially preferred, and as is seen from the results of Examples 1to 7 given in Table 1 hereinafter, it is preferred that the magnesiumoxide be added in an amount of 15 to 110 parts by weight, particularly30 to 110 parts by weight, especially particularly 60 to 90 parts byweight, per 100 parts by weight of the polymeric compound formed bypolymerizing a diene compound.

In addition to magnesium oxide, carbon black, finely divided silica andflowers of zinc (ZnO powder) have a certain effect, and the boilingwater resistance of the polymeric compound formed by polymerizing adiene compound can be improved by from 2 to 7 times by addition of thesecompounds. Carbon black has the advantage that the thermal absorption isimproved, but it has the disadvantage that the boiling water resistanceeffect of the primer becomes widely and uncontrollably variable. Forexample, the boiling water resistance varies in a broad range of 50 to300 hours in coated articles obtained by performing coating under thesame conditions. Finely divided silica has the advantage that it doesnot precipitate in solution, but it has the disadvantage that cracks arereadily formed in the primer coating under some conditions. Flowers ofzinc have a high specific gravity and a high sedimenting property.Accordingly, when the primer is employed in the form of a solution, itis difficult to employ flowers of zinc.

As is seen from the foregoing description, the effect of these othercompounds is much lower than that of magnesium oxide and they have someother disadvantages. Accordingly, they are much inferior to magnesiumoxide.

By the term "diene compound" referred to in the instant specification,there is meant polymerizable compounds having a conjugated double bond,such as butadiene and isoprene. As the film-forming polymers formed bypolymerizing such diene compounds, there can be mentioned, for example,homopolymers such as polybutadiene and natural rubber shown in Examples8 and 10 of Table 1 given hereinafter, and copolymers such as NBR(acrylonitrile-butadiene copolymer) shown in Example 9 of Table 1 givenhereinafter. Further, the polymeric compound includes alow-molecular-weight polymeric compound such as disclosed in JapanesePatent Publication No. 4951/72, namely, polybutadiene having numberaverage molecular weight of 1000 to 5000. A copolymer of butadiene isreferred to include at least 50 percent by weight of the butadiene unit.

Other components can optionally be incorporated in the primer of thisinvention. For example, when the primer is used in the form of asolution, in order to prevent sedimentation of magnesium oxide, therecan be added a precipitation-preventing agent or dispersing agent suchas finely divided silica, carbon black, metal salts of higher organicacid and organic compounds of monmorillonite. The amount of these agentsis from 0.1 to 20 parts by weight, per 100 parts by weight of the dienepolymer. Further, a dye or pigment can be added to the primer so as toprevent formation of an uncoated area and to color the primer,especially when spray coating method is employed.

The primer may contain additives that prevent the degradation of theprimer at room temperature, such as phenolic antioxidants.

When the primer is used in the form of a solution, there can be used anysolvents capable of dissolving the polydiene component. In order toprevent sedimentation or precipitation of magnesium oxide, it ispreferred to use a solvent having a higher specific gravity.

As is seen from the results of Examples 11 to 13 given in Table 1, theprimer of this invention is effective even for other thermoplastic resinpowders, in addition to nylon powder. Especially, the primer of thisinvention has a good adhesiveness to polyethylene which cannot be bondedto metals by any conventional bonding methods and it imparts a highlyimproved hot water resistance when a polyethylene powder is used. Theprimer of this invention has a very excellent effect to nyloncopolymers.

The primer of this invention is used for fluidized-bed dip-coating. Thefluidized-bed dip-coating referred to in the instant specificationincludes all coating techniques according to which a primer is coated ona metal to be coated, the primer-coated metal is subjected to apre-heating treatment and then a powder coating is applied. Accordingly,any known techniques such as electrostatic coating, electrostaticdip-coating and flame spray coating methods can be employed for applyinga powder paint to a prime-coated metal substrate.

The primer according to this invention is preferably employed in apowder coating operation and, in addition, is employed for bonding athermoplastic resin onto a metal substrate. The bonding is carried outby coating the metal substrate with the primer, pre-heating theprime-coated metal, then contacting it with a solid or moltenthermoplastic resin in order to melt the resin so that the resin forms acoating on the prime-coated metal substrate and then cooling to solidifythe resin coating. This bonding is suited, for instance, for coatingwire and joining metal pieces.

As is seen from the Examples given hereinafter, the primer of thisinvention can be used for other metals such as aluminum and copper, andalso for chemically treated metals. As the resin powder, there can beused powders of thermoplastic resins such as nylon, polyethylene andpolyvinyl chloride and thermosetting resins such as epoxy resins.

A primer as above mentioned, which comprises a diene polymer andmagnesium oxide, can attain the best effect when its coating thicknessis from 4 to 7 microns. It has been found that the above primersubstantially exerts its properties by reaction with oxygen in air andthat when the thickness of the primer coating is too large, air is notsufficiently diffused into the interior of the primer layer from thesurface thereof, so that the desired properties of the prime coating arenot fully attained.

More specifically, we investigated various additives to find substancescapable of promoting the reaction of the primer with oxygen, and it wasfound that certain organic peroxides and certain metal ions areeffective. Above all, cobalt ion and manganese ion are especiallyeffective for this purpose. Organic peroxides such as cumyl peroxide areeffective, but they drastically shorten the pot life. Further, othermetal ions, for example, ions of zinc, lead, copper and iron, areeffective, but their effect is much lower than that of cobalt andmanganese ions.

Another object of this invention is to provide a primer for powdercoating comprising a polymeric compound formed by polymerization of adiene compound and having incorporated therein, magnesium oxide andcobalt ion and/or manganese ion.

The employment of a primer comprising a diene polymer, magnesium oxideand cobalt ion and/or manganese ion expands the permissible range of thethickness of the primer coating to from 4μ to 35μ, whereas when thecobalt ion and/or manganese ion is not present, the permissible range isfrom 4μ to 7μ, and practical application of the primer is greatlyfacilitated according to this invention.

The cobalt ion and/or manganese ion referred to in the instantspecification is added in the form of a compound capable of beingdissolved homogeneously in the polymeric compound formed bypolymerization of a diene compound or in a solvent capable of dissolvingtherein such polymeric compound. As is seen from the results shown inthe Examples given hereinafter, it is preferred that the cobalt ion andmanganese ion be added in a total amount of 0.05 to 1.00 part by weight,especially 0.2 to 0.6 part by weight, per 100 parts by weight of thepolymeric compound formed by polymerization of a diene compound.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are diagrams illustrating the peeling test described inthe following Examples.

This invention will now be further described by reference to thefollowing illustrative Examples and Comparative Examples.

EXAMPLES 1 TO 13 AND COMPARATIVE EXAMPLES 1 AND 2

The primers, having the compositions set forth in Table 1, wererespectively prepared by mixing a polymeric compound, an additive and asolvent by using an attrition mill (manufactured by Mitsui MiikeSeisakusho K. K.). The primers were respectively dip-coated onto theprimer coated area of a mild steel plate (JIS G3141 D; 90 mm×73 mm×1 mm)as shown in FIG. 1, the coated steel plate was dried and then wassubjected to a pre-heating treatment for 4 minutes in a furnacemaintained at 350° C. The thickness of the primer coating was about 6μImmediately after the steel plate had been withdrawn from the furnace,it was dipped in thermoplastic powder (listed in Table 1) which was inthe state of being fluidized by air. The coated steel plate was thenplaced in a post-heat treatment furnace maintained at 240° C. for 2minutes and then cooled with water to obtain a coated steel plate.

The coating of the coated plate was cut with a knife to form cut linesreaching the metal surface as shown by the solid lines in FIG. 2, andthe coated plate was immersed in hot water maintained at 100° C. Peelingof the thermoplastic resin coating occurred in the area not coated withthe primer, and the peeling advanced toward the primer-coated area. Thetime required for the peeling length to extend 25 mm into the primercoated area, that is, to reach the chain-dotted line, was measured as"peeling time." The results are shown in Table 1.

                                      TABLE 1                                     __________________________________________________________________________                                         Thermo-                                         Primer Composition            plastic                                                                              Peeling                           Example                                                                              Polymer Compound                                                                        Additive  Solvent   Resin  Time                              No.    (parts by weight)                                                                       (parts by weight)                                                                       (parts by weight)                                                                       Powder (hours)                           __________________________________________________________________________    Comparative                                                                          polybutadiene                                                                           --        carbon tetra-                                                                           nylon 12                                                                             20-30                             Example 1                                                                            (molecular          chloride (800)                                                                          powder.sup.1                                    weight = 2000)                                                                (100)                                                                  Example 1                                                                            polybutadiene                                                                           light magnesium                                                                         carbon tetra-                                                                           nylon 12                                                                             30-50                                    (molecular                                                                              oxide (15)                                                                              chloride (1000)                                                                         powder.sup.1                                    weight = 2000)                                                                (100)                                                                  Example 2                                                                            polybutadiene                                                                           light magnesium                                                                         carbon tetra-                                                                           nylon 12                                                                              60-100                                  (molecular                                                                              oxide (30)                                                                              chloride (1200)                                                                         powder.sup.1                                    weight = 2000)                                                                (100)                                                                  Example 3                                                                            polybutadiene                                                                           light magnesium                                                                         carbon tetra-                                                                           nylon 12                                                                             200-300                                  (molecular                                                                              oxide (60)                                                                              chloride (1500)                                                                         powder.sup.1                                    weight = 2000)                                                                (100)                                                                  Example 4                                                                            polybutadiene                                                                           light magnesium                                                                         carbon tetra-                                                                           nylon 12                                                                             300-500                                  (molecular                                                                              oxide (70)                                                                              chloride (1500)                                                                         powder.sup.1                                    weight = 2000)                                                                (100)                                                                  Example 5                                                                            polybutadiene                                                                           light magnesium                                                                         carbon tetra-                                                                           nylon 12                                                                             200-300                                  (molecular                                                                              oxide (90)                                                                              chloride (1700)                                                                         powder.sup.1                                    weight = 2000)                                                                (100)                                                                  Example 6                                                                            polybutadiene                                                                           light magnesium                                                                         carbon tetra-                                                                           nylon 12                                                                             60-90                                    (molecular                                                                              oxide (110)                                                                             chloride (2000)                                                                         powder.sup.1                                    weight = 2000)                                                                (100)                                                                  Example 7                                                                            polybutadiene                                                                           light magnesium                                                                         carbon tetra-                                                                           nylon 12                                                                             10-20                                    (molecular                                                                              oxide (120)                                                                             chloride (2000)                                                                         powder.sup.1                                    weight = 2000)                                                                (100)                                                                  Example 8                                                                            polybutadiene                                                                           light magnesium                                                                         carbon tetra-                                                                           nylon 12                                                                             300-500                                  rubber.sup.2 (100)                                                                      oxide (70)                                                                              chloride (2500)                                                                         powder.sup.1                                              and fine silica.sup.3                                                         (3)                                                          Example 9                                                                            NBR.sup.4 (100)                                                                         light magnesium                                                                         chloroform (2500)                                                                       nylon 12                                                                             300-500                                            oxide (70)          powder.sup.1                                              and fine silica.sup.3                                                         (3)                                                          Example 10                                                                           natural rubber                                                                          light magnesium                                                                         carbon tetra-                                                                           nylon 12                                                                             300-400                                  (100)     oxide (70)                                                                              chloride (2500)                                                                         powder.sup.1                                              and fine silica.sup.3                                                         (3)                                                          Comparative                                                                          polybutadiene                                                                           --        carbon tetra-                                                                           polyethylene                                                                         15                                Example 2                                                                            (molecular          chloride (800)                                                                          type   days.sup.7                               weight = 2000)                copolymer                                       (100)                         powder.sup.6                             Example 11                                                                           polybutadiene                                                                           light magnesium                                                                         carbon tetra-                                                                           polyethylene                                                                         35                                       (molecular                                                                              oxide (70)                                                                              chloride (1500)                                                                         type   days.sup.7                               weight = 2000)                                                                          and fine silica.sup.3                                                                             copolymer                                       (100)     (3)                 powder.sup.6                             Example 12                                                                           polybutadiene                                                                           light magnesium                                                                         carbon tetra-                                                                           polyethylene                                                                         10                                       (molecular                                                                              oxide (70)                                                                              chloride (1500)                                                                         powder.sup.8                                                                         days.sup.7                               weight = 2000)                                                                          and fine silica.sup.3                                               (100)     (3)                                                          Example 13                                                                           polybutadiene                                                                           light magnesium                                                                         carbon tetra-                                                                           vinyl  25                                       (molecular                                                                              oxide (70)                                                                              chloride (1500)                                                                         chloride                                                                             days.sup.10                              weight = 2000)                                                                          and fine silica.sup.3                                                                             resin                                           (100)     (3)                 powder.sup.9                             __________________________________________________________________________     Notes:                                                                        .sup.1 Daiamide Z2073 manufactured by Daicel K. K.                            .sup.2 JSR BR01 manufactured by Nippon Gosei Gomu K. K.                       .sup.3 Aerosil 380 manufactured by Nippon Aerosil K. K.                       .sup.4 JSR N232 S manufactured by Nippon Gosei Gomu K. K.                     .sup.5 In Comparative Example 2 and Examples 11 to 13, since peeling did      not advance stepwise, the time required until the coating could be readil     peeled by a hand was measured as "peeling time".                              .sup.6 Admer N100 manufactured by Mitsui Sekiyu Kagaku K. K.                  .sup.7 Tested in hot water maintained at 80° C.                        .sup.8 Frosen manufactured by Seitetsu Kagaku K. K.                           .sup.9 Evaclad No. 1210 manufactured by Kansai Paint K. K.                    .sup.10 Tested in hot water maintained at 50° C.                  

EXAMPLES 14-30

The components shown in Table 2 were mixed and dispersed by using amixing and dispersing machine (Disper mill manufactured by HosokawaTekko K. K.) to form a primer.

The thus-formed primer was dip-coated on a mild steel plate (JIS G3141D; 90 mm×73 mm×1 mm) as shown in FIG. 1, and the steel plate was dried.Dipping was repeated to adjust the thickness of the primer layer. Theprimer thickness was measured by an electromagnetic film thicknessmeter. Then, the plate was subjected to the pre-heat treatment for 4minutes in a furnace maintained at 350° C. Immediately after the steelplate had been withdrawn from the furnace, it was dipped inthermoplastic powder which was in the state of being fluidized by air,and the steel plate was then placed in a heating furnace maintained at240° C. for 2 minutes and then was cooled with water to obtain a coatedsteel plate.

The coating of the coated plate was cut with a knife to form cut linesreaching the metal surface as shown in FIG. 2, and the coated plate wasimmersed in hot water maintained at 100° C. Peeling occurred at the areanot coated with the primer, and peeling advanced toward theprimer-coated area. The range of thickness of the respective primercoatings, that was effective to make longer than 15 days the timerequired for the peeling length to reach 25 mm in the primer coatedarea, is indicated by a thick solid line in Table 2.

                                      TABLE 2                                     __________________________________________________________________________                         Ion (part by             Resin                                  Components of Primer                                                                        weight)     Thickness of Primer (μ)                                                                 Powder                          __________________________________________________________________________     Referential Example Example 14 Example 15                                            100 parts by weight of polybutadiene.sup.1, 70 parts by weight of            agnesium oxide.sup.2, 3 parts by weight of Aerosil 380 and 1400               parts by weight trichloroethane 100 parts by weight                           of polybutadiene.sup.1, 70 parts by weight of magnesium                       oxide.sup.2, 3 parts by weight of Aerosil 380 and 1400 parts by               weight trichloroethane 100 parts by weight of polybutadiene.sup.1,            0 parts by weight of magnesium oxide.sup.2, 3 parts by weight of              Aerosil 380 and 1400 parts by weight trichloroethane                                         0 cobalt ion.sup.3 0.05 cobalt ion.sup.3 0.10                                             ##STR1##     nylon 12.sup.5 nylon                                                         12.sup.5 nylon 12.sup.5          Example 16 Example 17 Example 18                                                     100 parts by weight of polybutadiene.sup.1, 70 parts by weight of            agnesium oxide.sup.2, 3 parts by weight of Aerosil 380 and 1400               parts by weight trichloroethane 100 parts by weight                           of polybutadiene.sup.1, 70 parts by weight of magnesium                       oxide.sup.2, 3 parts by weight of Aerosil 380 and 1400 parts by               weight trichloroethane 100 parts by weight of polybutadiene.sup.1,            0 parts by weight of magnesium oxide.sup.2, 3 parts by weight of              Aerosil 380 and 1400 parts by weight trichloroethane                                         cobalt ion.sup.3 0.20 cobalt ion.sup.3 0.30 cobalt                           ion.sup. 3 0.40                                                                            ##STR2##     nylon 12.sup.5 nylon                                                         12.sup.5 nylon 12.sup.5          Example 19 Example 20 Example 21                                                     100 parts by weight of polybutadiene.sup.1, 70 parts by weight of            agnesium oxide.sup.2, 3 parts by weight of Aerosil 380 and 1400               parts by weight trichloroethane 100 parts by weight                           of polybutadiene.sup.1, 70 parts by weight of magnesium                       oxide.sup.2, 3 parts by weight of Aerosil 380 and 1400 parts by               weight trichloroethane 100 parts by weight of polybutadiene.sup.1,            0 parts by weight of magnesium oxide.sup.2, 3 parts by weight of              Aerosil 380 and 1400 parts by weight trichloroethane                                         cobalt ion.sup.3 0.50 cobalt ion.sup.3 0.60  cobalt                          ion.sup.3 0.80                                                                             ##STR3##     nylon 12.sup.5 nylon                                                         12.sup.5 nylon 12.sup.5          Example 22 Example 23 Example 24                                                     100 parts by weight of polybutadiene.sup.1, 70 parts by weight of            agnesium oxide.sup.2, 3 parts by weight of Aerosil 380 and 1400               parts by weight trichloroethane 100 parts by weight                           of polybutadiene.sup.1, 70 parts by weight of magnesium                       oxide.sup.2, 3 parts by weight of Aerosil 380 and 1400 parts by               weight trichloroethane 100 parts by weight of polybutadiene.sup.1,            0 parts by weight of magnesium oxide.sup.2, 3 parts by weight of              Aerosil 380 and 1400 parts by weight trichloroethane                                         cobalt ion.sup.3 1.00 manganese ion.sup. 4 0.05                              manganese ion.sup.4 (0.20)                                                                 ##STR4##     nylon 12.sup.5 nylon                                                         12.sup.5 nylon 12.sup.5          Example 25 Example 26 Example 27                                                     100 parts by weight of polybutadiene.sup.1, 70 parts by weight of            agnesium oxide.sup.2, 3 parts by weight of Aerosil 380 and 1400               parts by weight trichloroethane 100 parts by weight                           of polybutadiene.sup.1, 70 parts by weight of magnesium                       oxide.sup.2, 3 parts by weight of Aerosil 380 and 1400 parts by               weight trichloroethane 100 parts by weight of polybutadiene.sup.1,            0 parts by weight of magnesium oxide.sup.2, 3 parts by weight of              Aerosil 380 and 1400 parts by weight trichloroethane                                         manganese ion.sup.4  (0.40) manganese ion.sup.4                              (0.60) manganese ion.sup.4 (0.80)                                                          ##STR5##     nylon 12.sup.5 nylon                                                         12.sup.5 nylon 12.sup.5          Example 28 Example 29 Example 30.sup.6                                               100 parts by weight of polybutadiene.sup.1, 70 parts by weight of            agnesium oxide.sup.2, 3 parts by weight of Aerosil 380 and 1400               parts by weight trichloroethane 100 parts by weight                           of polybutadiene.sup.1, 70 parts by weight of magnesium                       oxide.sup.2, 3 parts by weight of Aerosil 380 and 1400 parts by               weight trichloroethane 100 parts by weight of polybutadiene.sup.1,            0 parts by weight of magnesium oxide.sup.2, 3 parts by weight of              Aerosil 380 and 1400 parts by weight trichloroethane                                          manganese ion.sup.4 (1.00) manganese ion(0.20) and                          cobalt ion(0.20) cobalt ion(0.35)                                                          ##STR6##     nylon 12.sup.5 nylon                                                         12.sup.5 poly- ethylene                                                       copolymer.sup.7                 __________________________________________________________________________     Notes:                                                                        .sup.1 Nisso Polybuta B3000 manufactured by Nippon Soda K. K. (number         average molecular weight = 3000).                                             .sup.2 Kyowa Magu #30 manufactured by Kyowa Kagaku Kogyo K. K.                .sup.3 Added in the form of cobalt naphthenate.                               .sup.4 Added in the form of manganese naphthenate.                            .sup.5 Daiamide Z2073 manufactured by Daicel K. K.                            .sup.6 The thickness of the primer layer in which the time required for       the peel length to reach 25 mm in a 3% aqueous solution of sodium chlorid     maintained at 60° C. was longer than 30 days is indicated by a         thick solid line in Table 2.                                                  .sup.7 Admer N100 manufactured by Mitsui Sekiyu Kagaku K. K.             

The embodiments of the invention in which an exclusive property or privilege is claimed as defined as follows:
 1. A primer composition for use in the coating of a resin powder onto a metal substrate, consisting essentially of a film-forming diene polymer dissolved in an inert organic solvent to form a solution, said solution having dispersed therein from 30 to 110 parts by weight of magnesium oxide particles, per 100 parts by weight of said diene polymer, said solution containing dissolved therein a cobalt compound, a manganese compound or a mixture of said compounds in an amount such that the solution contains from 0.05 to 1.0 part by weight, per 100 parts of said diene polymer, of cobalt ion, manganese ion or mixture thereof.
 2. A primer composition as claimed in claim 1 in which said magnesium oxide is light magnesium oxide, said primer composition containing from 0.2 to 0.6 part by weight of said cobalt ion, said manganese ion or mixture thereof, per 100 parts by weight of said diene polymer.
 3. A primer composition as claimed in claim 2 containing from 60 to 90 parts by weight of light magnesium oxide, per 100 parts by weight of said diene polymer.
 4. A primer composition as claimed in claim 1 in which said diene polymer is polybutadiene, natural rubber, or acrylonitrile-butadiene copolymer. 